A toner printer forms images by converting image data into printing instructions that define how much toner is to be applied to each portion of a receiver and by using the printing instructions to make a toner image. The toner image is transferred to a receiver and fused to form a toner print. During fusing, the toner is heated so that it spreads against the receiver to bond therewith.
The process of converting the image data into printing instructions assumes that the toner printer applies a consistent amount of toner in response to individual printing instructions. However, there are a wide variety of factors that can cause variations in the amount of toner that is applied to a receiver in response to printing instructions. These factors can include environmental factors such as ambient temperature and humidity, material variations such as variations in toner charging characteristics, and process variations such as wear and tolerances within the printer. Additionally, there are a variety of process set points such as primary charger set points, exposure set points, and toner concentration settings that can influence an amount of toner transferred to a receiver in response to a printing instruction.
Accordingly, toner printers typically include automatic process control systems that monitor the colors generated by a toner printer and that make adjustments to the set points used in the toner printer to ensure that the toner printer provides toner images having a consistent amount of toner in response to specific printing instructions.
In a conventional process control system a test patch is printed on a receiver according to a set of printing instructions that are expected to cause the test patch to have a particular color. The test patch is then fused and a reflective density of the test patch is measured. The measured reflective density is compared to an expected reflective density of the test patch and adjustments to printer set points are automatically made to correct any differences.
For example, in many toner printers, an in-line densitometer is used to make reflective density measurements test patches. An “in-line” densitometer refers to a densitometer that is mounted on the printer itself and which measures the reflective density of fused test patches on printed sheets moving through a paper path in the printer. Density measurements made by the in-line densitometer are transmitted to a digital color controller of the printer as the densitometer scans the moving sequence of test patches (which are typically a series of cyan, magenta, yellow, gray and black rectangles) on the printed test sheets. From the input provided by the in-line densitometer, a digital color controller in a toner printer can determine whether it is necessary to make adjustments in the amount of one or more toners applied in response to particular printing instructions.
FIG. 1 illustrates a conventional in-line densitometer 10 that measures reflection density. As is shown in FIG. 1, densitometer 10 has a light source 12 that emits a light L that is directed to illuminate a fused toner image 14 on a sheet 16. A portion of light L is absorbed by fused toner image 14 and sheet 16, a portion of light L is reflected as diffusely reflected light DRL and a portion of light L is reflected as specularly reflected light SRL that travels to light sensor 18.
FIG. 2 illustrates another example of a conventional in-line densitometer for measuring reflection density. In this example, densitometer 10 has light sensor 18 positioned to sense light that diffusely reflects from fused toner image 14 and sheet 16.
Conventional reflection type densitometry as illustrated in FIGS. 1 and 2 has a number of limitations. A first limitation is that reflection type densitometry cannot be accurately used to determine how much clear toner has have been fused to a receiver. This is because fused clear toner does not significantly impact the amount of light that reflects from the receiver and the reflective density measurements from an area having a large amount of fused clear toner do not differ significantly from reflective optical density measurements from an area having a relatively small amount of clear toner fused thereto.
A second limitation of reflective densitometry of the type that is illustrated in FIGS. 1 and 2 is that such conventional densitometry cannot be accurately used to measure how much unfused toner has been applied to a test patch of a receiver. There are a number of reasons for this. One reason for this is that unfused toner particles can be approximated as generally rounded objects that are positioned along the surface of a receiver. Therefore, toner particles reflect light in many different directions most of which are not in a path from a light source to a light sensor in a reflection density type of densitometer. When a reflection densitometer such as the one shown in FIG. 1 is used on an area having unfused toner, much of the light from the target area is reflected away from the light sensor and conclusions made based upon measurements made in this fashion can be misleading. Further, because toner particles rest on top of the receiver, light can be masked or trapped between the toner particles and the receiver creating optical effects that create uncertainty in as to whether differences in optical reflection measured made by a reflective densitometer of the type that is shown in FIG. 2 are indicative of differences in the amount of toner applied to a receiver or are indicative of such optical effects.
Additionally, it will be appreciated that unfused toner is disbursed over the surface area of receiver 26 in amounts that are calculated to form a particular color after the toner particles have been fused and spread so that the fused toner covers a greater portion of the receiver after fusing than before fusing. Therefore, any light received at a sensor from a test patch using conventional reflective densitometry will have a high proportion of light reflected from receiver 26. The light that is reflected by toner particles will generally be darker than the light that is reflected by the receiver. Further, the toner reflected light has lower intensity than the receiver reflected light. These characteristics of such reflected light limit the reliability with which a densitometer can discriminate between different amounts of unfused toner in a test patch.
Accordingly, conventional densitometers can only provide process control signals after a print has been printed and fused. This creates additional limitations in that process control determinations can only be made after the printing of an image is complete. Thus, where corrections are necessary, at least one print evidencing the need for such corrections must be made and recycled. Additionally, the measurements made by the densitometer can be impacted both by the fusing process and by the amount of toner in an area that is measured and it can be unclear whether corrections are to be made to set points for fusing or to the amounts of toner applied to a receiver.
For these reasons, conventional reflective densitometry measurements cannot be applied reliably to the measurement of unfused toner amounts and there remains a need in the art for an in-line system that can be used to reliably measure amounts of unfused toner that are applied to a receiver by a toner printer. Further, to reduce printer complexity and cost, it is desirable that such an in-line system be inexpensive and of efficient design while still overcoming all of the aforementioned disadvantages associated with prior art designs.